Semiconductor device

ABSTRACT

A semiconductor device including: a first conductivity type n-type drift layer; a second conductivity type VLD region which is formed on a chip inner circumferential side of a termination structure region provided on one principal surface of the n-type drift layer and which is higher in concentration than the n-type drift layer; a second conductivity type first clip layer which is formed on a chip outer circumferential side of the VLD region so as to be separated from the VLD region and which is higher in concentration than the n-type drift layer; and a first conductivity type channel stopper layer which is formed on a chip outer circumferential side of the first clip layer so as to be separated from the first clip layer and which is higher in concentration than the n-type drift layer. Thus, it is possible to provide a semiconductor device having a stable and high breakdown voltage termination structure in which the length of a termination structure region is small as well as the immunity to the influence of external charge is high.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a termination structure forsemiconductor devices, especially, high breakdown voltage semiconductordevices (such as diodes, IGBTs or MOSFETs) used in power converters orthe like.

2. Description of the Related Art

High breakdown voltage discrete power devices play a central role inpower converters. The devices include elements such as insulated gatebipolar transistors (IGBTs), and metal oxide semiconductor field-effecttransistors (MOSFETs). Each of the devices consists of an active portionwhich controls a current, and a termination structure (or edgestructure) which ensures breakdown voltage. Although it is a matter ofcourse that the breakdown voltage of an ideal termination structure ishigher than that of the active portion, the breakdown voltage of thetermination structure must endure the influence of external ions. Forexample, a discrete element formed in such a manner that a powersemiconductor chip is sealed with a resin is a form type of powersemiconductor device (IGBT, MOSFET, diode, etc.) products. A powermodule formed in such a manner that a power semiconductor chip is housedin a housing or a power semiconductor chip and a control circuit chipare housed in a housing is another form type. External ions always existin a peripheral environment of power converters using these powersemiconductor devices. The external ions come from a sealing material(gel, epoxy resin, etc.) used in packages of these discrete elements orpower modules and reach a surface of a termination structure. Theexternal ions which have reached the surface of the terminationstructure pass through a passivation layer of the device and reach anupper portion of a silicon region of the device to cause deteriorationof the breakdown voltage of the termination portion. Accordingly, notonly the module/package material and the passivation material of theelement must have functions of suppressing the influence of externalions but also the configuration of the termination structure region perse must be designed as a structure capable of suppressing the influenceof external charge (external ions), that is, to have resistance thereto.

A guard ring structure (hereinafter simply referred to as guard ringstructure) flanked with conductive field plate has been heretofore usedwidely as a termination structure. For example, the guard ring structurehas been disclosed in B. Jayant Baliga “Fundamentals of PowerSemiconductor Devices” (US), first edition, Springer Science+BusinessMedia, 2008, p. 137. FIG. 41 is a sectional view showing the guard ringstructure. A p⁺ guard ring 20 a which is connected to an emitterelectrode 11 and p⁺ guard rings 20 b to 20 f which are not connected tothe emitter electrode 11 so as to be electrically floating are formed ina termination structure region 33. A channel stopper layer 6 is formedat an outer circumferential end of the device (at a right end of thetermination structure region 33 in FIG. 41) so as to be distant from thep⁺ guard ring 20 f. Conductive field plates 21 a to 21 e are connectedto the p⁺ guard rings 20 b to 20 f respectively and a field plate(stopper field plate 15) of the channel stopper layer 6 is connected tothe channel stopper layer 6. This field plate-including guard ringstructure has excellent characteristic in charge resistance because theinfluence of external charge coming from the outside on electriccharacteristic is a little. In the termination structure region 33,silicon oxide films such as separation oxide films 2 or interlayerinsulating films 8 formed on a surface are covered with the field plates21 a to 21 e to thereby prevent movement of an equipotential surfacegenerated when external charge reaches the vicinity of the oxide films.Moreover, because potential in the vicinity of each of the p⁺ guardrings 20 a to 20 f follows the equipotential surface distribution of adepletion layer spread inside silicon, potential of each of the p⁺ guardrings 20 a to 20 f per se is stable. Because the stable potentialdetermines potential of each of the field plates 21 a to 21 e, thesilicon oxide film exposed from the surface of the termination structureregion is prevented more sufficiently from being affected by externalcharge.

A structure for preventing more sufficiently the influence of externalcharge coming from the outside has been disclosed in U.S. Pat. No.6,445,054 (FIG. 42). When external charge is present in the surface ofthe termination structure region 33, charge is induced between thesurface of the n-type drift layer 1 confronting the separation oxidefilm 2 and the separation oxide film 2 and between the surface of then-type drift layer 1 and the interlayer insulating film 8. The n-channelstopper layer 6 equipotential to a collector electrode 14 iselectrically connected to the emitter electrode 11 by the charge, sothat a large leakage current is generated or the breakdown voltage isdeteriorated. To prevent the electrical connection, n-type channelstopper layers 23 higher in concentration than the n-type drift layerare formed so as to be adjacent to the active region sides of the p⁺guard rings 20 b to 20 f, respectively. Similarly, p-type channelstopper layers 22 are formed so as to be adjacent to the device outercircumferential sides of the p⁺ guard rings 20 a to 20 f, respectively.For example, when positive external charge comes into the surface of thetermination structure region, an electron storage layer where electronsare stored as negative charge is formed in an SiO₂/Si interface of then-type drift layer 1. Therefore, electrical connection between thecollector electrode and the emitter electrode by the electron storagelayer is suppressed by the p-type channel stopper layers 22. On theother hand, when negative external charge comes into the surface of thetermination structure region, a hole storage layer where holes arestored as positive charge is formed in the SiO₂/Si interface of then-type drift layer 1. Therefore, electrical connection between thecollector electrode and the emitter electrode by the hole storage layeris suppressed by the n-type channel stopper layers 23.

On the other hand, a structure shown in FIG. 43 has been disclosed inJP-A-2003-23158 in order to shorten the length of the terminationstructure region. According to FIG. 43, a RESURF (Reduced SurfaceElectric Field) layer 38 having a RESURF effect is formed in thetermination structure region so as to be adjacent to a p⁺ layerconnected to the emitter electrode 11. The RESURF layer 38 is a layersufficiently lower in concentration than a general guard ring layer, sothat the RESURF layer 38 can have a shorter distance than that of theguard ring structure to relax electric field intensity.

A termination structure dubbed VLD (Variation of Lateral Doping) hasbeen disclosed in JP-A-Sho-61-84830 (FIG. 40). That is, a p-type dopant(such as boron) is imported and diffused into a terminal structureregion through a large number of opening portions in the separationoxide film 2. As shown in FIG. 40, lateral diffusion portions of p-typediffusion layers 17 a to 17 d adjacent to one another overlap with oneother. The widths of the opening portions are reduced in a direction offrom the chip inner circumferential side to the chip outercircumferential side, and the ratio of each opening portion to theseparation oxide film is reduced. Therefore, the p-type diffusion layers17 a to 17 d are formed so that both diffusion depth and concentrationare reduced in a direction of from the chip inner circumferential sideto the chip outer circumferential side. A pn junction between the p-typediffusion layers 17 a to 17 d and the n-type drift layer 1 is generallyformed as a distribution of envelop curves of the p-type diffusionlayers 17 a to 17 d or wavy curves based on the envelope curves. Theaforementioned termination structure shown in FIG. 40 is called VLDstructure. A region where the p-type diffusion layers 17 a to 17 doverlap with one another is called VLD region 17. Because the lateraldiffusion portions of the adjacent p-type diffusion layers 17 a to 17 doverlap with one another, the length of the termination structure regionis substantially equal to the length of the RESURF layer, so that theVLD structure becomes sufficiently shorter than the guard ringstructure.

In the aforementioned guard ring structure, the length occupied by thetermination structure region in the chip generally becomes large becauseof the width of each p⁺ guard ring layer per se and the number of p+guard ring layers arranged. For this reason, the ratio of the areaoccupied by the termination structure region in the chip becomes largewhen the guard ring structure is used for a high breakdown voltage (e.g.1700 V or higher) device in which the termination structure region mustbe long inevitably or when the guard ring structure is used for a smallcurrent purpose in which the area of a device chip becomes small. As aresult, the number of chips fractionated from one silicon wafer isreduced to cause a problem that chip cost increases.

In the RESURF structure, it is necessary to distribute potential ofsilicon surface (equipotential surface) equally. For this reason, thesilicon oxide films cannot be covered with the field plates, so that theequipotential surface distribution shape of the RESURF structure portionvaries sensitively according to external charge to thereby lower thebreakdown voltage.

In the VLD structure, the concentration of the p-type diffusion layers(hereinafter referred to as VLD region) of the termination structureregion can be set to be relatively higher than that of the RESURFstructure, so that in this respect, the VLD structure is hardly affectedby external charge. However, the stability of the VLD structure isinferior to that of the guard ring structure yet. As described above,when external charge comes flying, the equipotential surfacedistribution shape in the termination structure region changes. For thisreason, the electric field intensity distribution in the vicinity of thesemiconductor surface of the termination structure region changes, sothat the position of maximum electric field intensity shifts to the chipinner circumferential side or outer circumferential side in accordancewith the polarity of external charge. In the case of a p-type VLDstructure, the maximum electric field intensity according to positiveexternal charge moves only in the inside of the VLD structure becausethe depletion layer shifts to the chip inner circumferential side. Thatis, the VLD structure can absorb the influence of positive externalcharge. However, one of the maximum electric field intensity maximaaccording to negative external charges moves to the outercircumferential side of the VLD region because the depletion layershifts to the chip outer circumferential side. As a result, the electricfield intensity in the VLD region is reduced. The voltage allowed to bewithstood by the voltage breakdowning structure is a laterallyintegrated value of electric field intensity. As a result of shifting ofthe depletion layer, the integrated value in the VLD structure portionis reduced so that the voltage change cannot be absorbed. For thisreason, even in the VLD structure, the breakdown voltage according tonegative external charge is reduced.

SUMMARY OF THE INVENTION

In consideration of the above described prior arts, object of theinvention is to provide a semiconductor device having a stable and highbreakdown voltage termination structure in which a termination structureregion is short as well as highly immune to the influence of externalcharge

To solve the problem and achieve the object of the invention, theinvention provides a semiconductor device including: a first electrodeformed on one principal surface of a first conductivity typesemiconductor substrate; a second electrode formed on the otherprincipal surface of the semiconductor substrate; a second conductivitytype base layer formed on the one principal surface of the semiconductorsubstrate so as to be connected to the first electrode; a secondconductivity type VLD region provided on an outer circumferential sideof the base layer; and a first or second conductivity type stopper layerprovided on an outer circumferential side of the VLD region so as to beseparated from the VLD region; wherein: the semiconductor device furtherincludes a second conductivity type first clip layer which is providedbetween the VLD region and the stopper layer so as to be separated fromthe VLD region and the stopper layer and which is higher inconcentration than the semiconductor substrate.

The structure of the semiconductor device according to the invention ischaracterized in that the second conductivity type first clip layer isprovided on the outer circumferential side of the VLD region. Accordingto this structural characteristic, electric field intensity in thevicinity of the first clip layer can be increased even when a depletionlayer is spread to the chip outer circumferential side because ofexternal charge coming flying on an upper surface of the terminationstructure region, so that a lowering of a bearable voltage (breakdownvoltage) can be suppressed.

The invention is further characterized in that a spacing region isprovided for separating the first clip layer and the stopper layer fromeach other. According to this characteristic, maximum electric fieldintensity is fixed in the vicinity of the first clip layer even when theplace of maximum electric field intensity shifts to the outercircumferential side of the VLD region. Moreover, the provision of thespacing region permits a high voltage to be endured by the spacingregion. As a result, lowering of the breakdown voltage can be preventedagainst external charge with both positive and negative polarities.

Preferably, a depletion layer spreading from a joint surface between thefirst clip layer and the first conductivity type semiconductor substrateinto the first clip layer when a voltage equivalent to an avalanchebreakdown voltage of the semiconductor device is applied between thesecond electrode and the first electrode is thicker than a chargeneutral region which is a remaining non-depleted portion of the firstclip layer.

Assume now that the concentration of the first clip layer is so highthat no depletion occurs. Then, when the depletion layer (equipotentialsurface) shifts to the outer circumferential side of the VLD regionbecause of external charge, the first clip layer prevents the depletionlayer from spreading. For this reason, a high electric field region isgenerated locally in the vicinity of the first clip layer, so that anavalanche current is generated to reduce the breakdown voltage. Toprevent high electric field intensity from being generated locally inthe vicinity of the first clip layer, equipotential surface density inthe inside of the first clip layer may be reduced. It is thereforepreferable that the depletion layer is spread into the first clip layeras sufficiently as possible. It is particularly preferable that thedepletion layer spreading into the first clip layer is deeper than aremaining non-depletion portion of the first clip layer. In this manner,electric field intensity in the first clip layer can be absorbed sosufficiently that high electric field intensity can be prevented frombeing generated locally.

Preferably, the semiconductor device further includes a firstconductivity type second clip layer which is provided on an outercircumferential side of the VLD region and on an inner circumferentialside of the first clip layer so as to be separated from the VLD regionand which is higher in concentration than the semiconductor substrate.

When negative external charge reaches an upper surface of thetermination structure region, a channel of carriers (holes) is formed ina semiconductor surface in an interface between an oxide film forprotecting the upper surface of the semiconductor region and thesemiconductor region. A leakage current path is generated between thefirst electrode and the second electrode through the carrier channel, sothat this causes a leakage current. On the contrary, the channel ofholes can be eliminated by the second clip layer.

Chief ones of preferred means concerned with the invention will bedescribed. Other means will be described later in “DETAILED DESCRIPTIONOF THE INVENTION”.

Preferably, the first clip layer is deeper than the second clip layer.

When the depletion layer spreads in the termination structure regionfrom the chip inner circumferential side to the chip outercircumferential side, the depletion layer is not stopped by the secondclip layer and can reach the first clip layer if the first clip layer isdeeper than the second clip layer.

Preferably, the second clip layer is adjacent to the first clip layer.

In this case, the depletion layer little collides with the second cliplayer and can reach the first clip layer, so that electric fieldintensity can be relaxed.

Preferably, a first field plate is formed on a surface of the first cliplayer.

In this case, surface potential of the first clip layer is fixed overthe whole of the region where the first field plate is formed.Accordingly, equipotential surface change based on external chargecoming flying can be reduced more sufficiently.

Preferably, the length of the first field plate toward an innercircumferential side of the first clip layer is larger than the lengthof the first field plate toward an outer circumferential side of thefirst clip layer.

Spreading of the depletion layer into the first clip layer on the chipinner circumferential side is larger than that on the chip outercircumferential side. For this reason, electric field intensity in thevicinity of the chip inner circumferential side of the first clip layerincreases. Accordingly, the length on the chip inner circumferentialside of the first field plate should be set to be larger than the lengthon the chip outer circumferential side of the first field plate, by thisway the electric field intensity in the vicinity of the chip innercircumferential side of the first clip layer can be relaxed.

Preferably, the second clip layer is covered with the first field platethrough an insulating film.

The second clip layer is of the same first conductivity type as that ofthe semiconductor substrate and higher in concentration than thesemiconductor substrate. For this reason, when the depletion layerreaches the second clip layer, electric field intensity increases to avery large value. When the second clip layer is formed on the outercircumferential side of the inner circumferential side end portion ofthe first field plate, the second clip layer is covered with the firstfield plate. For this reason, the depletion layer does not reach thesecond clip layer, so that the local increase of electric fieldintensity can be prevented.

Preferably, an outer circumferential side end portion of the VLD regionis located on an outer circumferential side of an outer circumferentialside end portion of the first electrode.

Relaxation of electric field intensity in the VLD region is obtainedwhen an equipotential surface is brought out of the protection oxidefilms formed on the surface of the VLD region. It is thereforepreferable that the outer circumferential side end portion of the firstelectrode is provided on an inner circumferential side of the outercircumferential side end portion of the VLD region.

Preferably, a second field plate is provided in the stopper layer sothat the first field plate is separated from the second field plate.

When the first field plate and the second field plate are separated fromeach other in the upper surface of the spacing region, equipotentialsurfaces are distributed to the spacing region. As a result, the voltagecan be endured by the spacing region even when the depletion layershifts to the chip outer circumferential side as described above.

As described above, in accordance with the invention, it is possible toprovide a semiconductor device having a stable and high breakdownvoltage termination structure in which the length of a terminationstructure region is so small that the influence of external charge isvery little.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing important part of a semiconductordevice according to an embodiment of the invention;

FIG. 2A is a sectional view showing important part of the semiconductordevice according to this embodiment;

FIGS. 2B to 2D are schematic views showing operation of thesemiconductor device according to this embodiment;

FIG. 3 is a sectional view showing important part of a semiconductordevice according to an embodiment of the invention;

FIGS. 4A and 4B are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 5A to 5E are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 6A to 6D are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 7A to 7D are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIG. 8 is a sectional view of important part showing a process ofproducing the semiconductor device according to the embodiment of theinvention;

FIGS. 9A to 9D are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 10A to 10E are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 11A and 11B are sectional views of important part showing aprocess of producing the semiconductor device according to theembodiment of the invention;

FIGS. 12A and 12B are sectional views of important part showing aprocess of producing the semiconductor device according to theembodiment of the invention;

FIGS. 13A to 13D are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 14A to 14C are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 15A to 15C are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIG. 16 is a sectional view of important part showing a process ofproducing the semiconductor device according to the embodiment of theinvention;

FIGS. 17A to 17C are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 18A to 18C are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 19A to 19C are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 20A to 20C are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIG. 21 is a sectional view of important part showing a process ofproducing the semiconductor device according to the embodiment of theinvention;

FIG. 22 is a characteristic graph showing electric characteristic of thesemiconductor device according to the embodiment of the invention andelectric characteristic of a semiconductor device according to therelated art;

FIG. 23 is a characteristic graph showing electric characteristic of thesemiconductor device according to the embodiment of the invention andelectric characteristic of the semiconductor device according to therelated art;

FIG. 24 is a sectional view showing important part of a semiconductordevice according to an embodiment of the invention;

FIG. 25 is a sectional view showing important part of a semiconductordevice according to an embodiment of the invention;

FIG. 26 is a sectional view of important part showing a process ofproducing the semiconductor device according to the embodiment of theinvention;

FIGS. 27A to 27D are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIG. 28 is a characteristic graph showing electric characteristics ofsemiconductor devices according to embodiments of the invention andelectric characteristics of semiconductor devices according to therelated art;

FIG. 29 is a graph showing doping concentration distributions of thesemiconductor devices according to the embodiments of the invention;

FIGS. 30A and 30B are characteristic graphs showing electriccharacteristics of the semiconductor devices according to theembodiments of the invention;

FIG. 31 is a sectional view showing important part of the semiconductordevice according to the embodiment of the invention;

FIG. 32 is a characteristic graph showing electric characteristic of thesemiconductor device according to the embodiment of the invention;

FIG. 33 is a sectional view showing important part of the semiconductordevice according to the embodiment of the invention;

FIG. 34 is a characteristic graph showing electric characteristic of thesemiconductor device according to the embodiment of the invention;

FIG. 35 is a sectional view showing important part of the semiconductordevice according to the embodiment of the invention;

FIG. 36 is a sectional view of important part showing a process ofproducing the semiconductor device according to the embodiment of theinvention;

FIG. 37 is a sectional view of important part showing a process ofproducing the semiconductor device according to the embodiment of theinvention;

FIGS. 38A to 38E are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIGS. 39A to 39D are sectional views of important part showing a processof producing the semiconductor device according to the embodiment of theinvention;

FIG. 40 is a sectional view showing important part of a semiconductordevice according to the background art;

FIG. 41 is a sectional view showing important part of a semiconductordevice according to the background art;

FIG. 42 is a sectional view showing important part of a semiconductordevice according to the background art; and

FIG. 43 is a sectional view showing important part of a semiconductordevice according to the background art.

DETAILED DESCRIPTION OF THE INVENTION

Although a first conductivity type and a second conductivity type willbe regarded as an n type and as a p type, respectively, the inventioncan be operated equally even when the n type and the p type are replacedwith each other. Although the terms “device”, “element”, “semiconductorchip” and simply “chip” are used for a semiconductor device in thisspecification, each of the terms means a semiconductor device producedas a chip and indicates the same object. Although the device accordingto the invention will be described while an insulated gate bipolartransistor (IGBT) is taken as an example, the device according to theinvention can be applied also to a well-known unipolar device such as aninsulated gate transistor (MOSFET), or a well-known p-i-n diode.

A first clip layer and a second clip layer will be hereinafter referredto as “clip p layer” and “clip n layer” respectively based on thepolarity relation between the p type and the n type.

The term “wafer” means a silicon substrate which has not been fragmentedinto chips yet.

In a semiconductor chip, a region where a first electrode (hereinafterreferred to as emitter electrode) is formed so that a current can beapplied to the region is called “active region”. Then, a chip outercircumferential end portion in a region where the emitter electrode isin contact with the semiconductor substrate (hereinafter referred to as“n-type drift layer”) or the like is defined as an outer circumferentialend portion of the active region and called as “active end portion”. Aregion which ranges from the active end portion to an outercircumferential end portion of the chip and which is a structure portionfor relaxing electric field intensity generated in a chip surface when avoltage is applied to the element is called “termination structureregion”.

For example, the expression “1.0E12/cm²” is used in description ofconcentration or the like. This means 1.0×10¹²/cm². The symbol “+” (“−”)written in the right of each layer (p layer or n layer) shown in eachdrawing means that the impurity concentration of this layer isrelatively higher (lower) than that of any other layer.

Specifically, the term “VLD region” means the following structure. Thatis, second conductivity type diffusion layers are formed over thetermination structure region in a range of from the active end portionto the outer circumferential end of the chip so that lateral diffusion(diffusion in directions parallel to the chip surface) portions of thesecond conductivity type diffusion layers overlap with one another. Sucha region where the second conductivity type diffusion layers arecollected is called “VLD region”. Typically, the VLD region has astructure in which the diffusion depth of the VLD region is deepest inthe vicinity of the active end portion and becomes shallow graduallytoward the outer circumference of the chip.

A basic structure of the invention will be described with reference toFIG. 1.

FIG. 1 is a sectional view of important part showing a basic structureof a semiconductor device according to the invention.

An active region 31 and a termination structure region 33 are formed onan upper surface of an n-type drift layer 1 which is an n-typesemiconductor substrate. For example, a so-called MOS gate structure oftrench type metal (conductive polysilicon)-insulator (silicon oxidefilm)-semiconductor will be described as the active region 31 in thisembodiment. A p-type base layer 5 higher in concentration than then-type drift layer 1 is provided so as to be in contact with an emitterelectrode 11 in an upper surface of the active region 31 through p⁺regions 7, 7 a and 7 b to reduce contact resistance. Trenches (grooves)are formed so that each trench passes through the p-type base layer 5from a surface of the device being in contact with the emitter electrode11 and reaches the n-type drift layer 1. Conductive gate polysilicon 4 ais embedded inside the trench through a gate oxide film 3. An n⁺ emitterlayer 6 b is formed on a side wall of the trench so as to be in contactwith the emitter electrode. Besides the trench type MOS gate structure,a well-known planar type MOS gate structure may be used. A region wherethe n⁺ emitter region 6 b is not formed may be suitably provided in anMOS gate structure of a portion of the active region 31 near thetermination structure region 33. A wave mark is written at the left endof an arrow indicating the active region 31 in FIG. 1. The wave markmeans that a plurality of MOS gate structures, etc. for forming theactive region of the device are formed in the left of the left end ofthe device sectional view shown in FIG. 1.

In addition to the p-type base layer 5 being in contact with the emitterelectrode 11, a p-type diffusion layer 17 a lower in concentration thanthe p-type base layer 5 and deeper in diffusion than the p-type baselayer 5 is provided in an active end portion 32 without provision of anyMOS gate. The p-type diffusion layer 17 a is provided as an activeregion side (chip inner circumferential) end portion of the VLD region17. Although there is a possibility that the p-type diffusion layer 17 awill be shallower in diffusion than the p-type base layer 5, theconcentration of the p-type diffusion layer 17 a is lower than that ofthe p-type base layer 5 even in this case.

A plurality of p-type diffusion layers 17 a to 17 d are formed in theupper surface of the n-type drift layer 1 in a range of from the activeend portion 32 to the outer circumference of the chip. The respectivewidths of the p-type diffusion layers 17 a to 17 d are formed inaccordance with the widths of opening portions 35 of a separation oxidefilm 2. That is, the distance between the respective opening portions 35of the p-type diffusion layers 17 a to 17 d is reduced gradually towarda chip outer circumferential end 46 located in the outer circumferentialend portion of the chip. The p-type diffusion layers 17 a to 17 dadjacent to one another are provided so that lateral diffusion portions34 (portions where p-type dopant is diffused in parallel to the chipsurface from each opening portion 35) overlap with one another. When theconcentration of the p-type diffusion layer 17 a nearest to the activeportion 31 is set to be highest in the VLD region 17, the width of theseparation oxide film 2 located on the lateral diffusion portions 34overlapping with one another is 6.0 μm. The width of each openingportion 35 of the p-type diffusion layer 17 a is 40.5 μm. Successively,in a region where the p-type diffusion layer 17 b is formed, the widthof each opening portion 35 is, for example, 10.5 μm. Successively, in aregion where the p-type diffusion layer 17 c is formed, the width ofeach opening portion 35 is, for example, 5.5 μm. As to the p-typediffusion layer 17 d in a region nearest to a VLD end portion 44, thewidth of each opening portion 35 is, for example, 2.0 μm.

The diffusion depths of the p-type diffusion layers 17 a to 17 d arereduced gradually or stepwise in a direction of from the p-typediffusion layer 17 a on the chip inner circumferential side toward thep-type diffusion layer 17 d on the outer circumferential side. Theaverage concentrations of the p-type diffusion layers 17 a to 17 d arereduced gradually or stepwise in a direction of from the p-typediffusion layer 17 a on the chip inner circumferential side toward thep-type diffusion layer 17 d on the outer circumferential side.Incidentally, the term “average concentration” means a depth-directiontotal impurity concentration per unit area or an impurity concentrationper unit area integrated in a depth direction. For example, the averageconcentration is expressed in atoms/cm². The gradients of the averageconcentration distributions of the p-type diffusion layers 17 a to 17 dcan be adjusted based on the widths of the opening portions 35 in theseparation oxide film 2 of the VLD region 17. That is, as will bedescribed in the production method, the separation oxide film 2 servesas a shield mask when the p-type dopant is ion-implanted. For thisreason, in a region where the remaining width of the separation oxidefilm 2 is small and the width of each opening portion 35 is large as inthe p-type diffusion layer 17 a, the average acceptor concentration ofthe region becomes sufficiently close to the dose quantity of theimplanted p-type dopant. On the other hand, in a region where theremaining width of the separation oxide film 2 is large and the width ofeach opening portion 35 is small as in the p-type diffusion layer 17 d,the average acceptor concentration of the region becomes sufficientlylower than the dose quantity of the implanted p-type dopant. Forexample, the dose quantity of boron is kept constant over the wafersurface. In the p-type diffusion layer 17 b in FIG. 1, the ratio of thearea of each opening portion 35 to the area of the separation oxide film2 adjacent to the opening portion 35 and serving as a mask shieldingregion (hereinafter referred to as aperture ratio) is, for example, 4:1.The average total impurity quantity (hereinafter referred to aseffective dose quantity) of the p-type diffusion layer 17 b on thisoccasion is 80% of the dose quantity of the ion-implanted p-type dopant.When the aperture ratio in the p-type diffusion layer 17 c is 1:1likewise, the effective dose quantity of the p-type diffusion layer 17 bis 50% of the ion-implantation dose quantity. When the aperture ratio inthe p-type diffusion layer 17 d is 1:9 likewise, the effective dosequantity of the p-type diffusion layer 17 b is 10% of theion-implantation dose quantity. In this manner, the lateralconcentration distribution of the VLD region 17 can be adjusted. As theeffective dose quantity becomes smaller, the VLD region 17 becomesshallower gradually toward the chip outer circumferential side becausethe diffusion depth of dopant in the same heat budget becomes smaller.The region formed from the p-type diffusion layers 17 a to 17 d is theVLD region 17. The upper surface of the VLD region 17 is covered withthe separation oxide film 2 and an interlayer insulating film 8.

A clip p layer 17 e distant by a length L_(sC) from the VLD region 17and higher in concentration than the n-type drift layer 1 is formed onthe chip outer circumferential side of the VLD region 17. An n-typechannel stopper layer 6 distant from the clip p layer 17 e and higher inconcentration than the n-type drift layer 1 is formed on the chip outercircumferential side of the clip p layer 17 e. Incidentally, the channelstopper layer 6 may be of a p type as long as the channel stopper layer6 is higher in concentration than the n-type drift layer 1. Specificstructure parameters of the p-type diffusion layers 17 a to 17 d and theclip p layer 17 e in the VLD region 17 and specific numerical valuesthereof will be described in Table 1. Incidentally, the p-type diffusionlayers 17 a to 17 d in the VLD region 17 are repeatedly shown in FIG. 40for describing the background art and with the same condition as that inFIG. 40.

TABLE 1 STRUCTURAL PARAMETER OF VLD REGION IN EMBODIMENT 1 ANDBACKGROUND ART REGION 17a 17b 17c 17d 17e DEPTH (μm) 6 6 6 5.8 5.8 WIDTH(μm) 42.5 12.5 7.5 5.5 10.7 SURFACE 7.4E+15 7.4E+15 7.2E+15 5.6E+155.6E+15 CONCENTRATION (cm⁻³) THE NUMBER OF STEPS 1 8 4 9 1

In the leftmost column of Table 1, “region” designates the referencenumeral described in FIG. 1. “Depth” designates a diffusion depth of apn junction 43 in each of the p-type diffusion layers 17 a to 17 d.“Width” designates a length of a region in each of the p-type diffusionlayers 17 a to 17 d. For example, the length of the p-type diffusionlayer 17 b is a distance from the active region 31-side end portion ofthe opening portion 35 of the p-type diffusion layer 17 b to the chipouter circumferential side end portion of the chip outer circumferentialside opening portion 35 of the p-type diffusion layer 17 b. “Surfaceconcentration” designates a p-type acceptor concentration in the chipsurface in each of the p-type diffusion layers 17 a to 17 d. “The numberof steps” designates the number of opening portions in each of thep-type diffusion layers 17 a to 17 d. The background art designates aVLD structure with provision of neither clip p layer 17 e nor clip nlayer 6 a as shown in FIG. 40.

The emitter electrode 11 is connected to the p-type base layer 5 throughthe contact p⁺ regions 7, 7 a and 7 b higher in concentration than theVLD region 17. In the VLD region 17, the p-type diffusion layers 17 aand 17 b are partially connected to the emitter electrode 11 through thep⁺ regions 7, 7 a and 7 b and the p-type base layer 5. A gatepolysilicon runner 4 b which will be described later is formed on anupper surface of the p-type diffusion layer 17 a wedged between the p⁺regions 7 a and 7 b, through a gate oxide film 3 formed on the uppersurface of the p-type diffusion layer 17 a.

The emitter electrode 11 is provided with a roof which has a length MFP1and which is formed in a range of from a portion connected to the p⁺region 7 b toward the chip outer circumferential side. A clip fieldplate 15 a is connected to an upper surface of the clip p layer 17 ethrough the p⁺ region 7 b higher in concentration than the clip p layer17 e. The clip field plate 15 a is formed so that the length MFP4 a of achip inner circumferential side roof is larger than a length MFP4 b of achip outer circumferential side roof. A stopper field plate 15 isprovided on an upper surface of the channel stopper layer 6 so as to beseparated from the clip field plate 15 a. Here, the region where theclip field plate 15 a and the stopper field plate 15 are separated fromeach other is referred to as spacing region 40. The length of the chipinner circumferential side roof of the stopper field plate 15 isreferred to as MFP2. A passivation film 13 is formed for protecting apart of the active region 31 and the upper surface of the terminationstructure region 33.

A well-known n-type field stop layer 9 is formed on a lower surface sideof the n-type drift layer 1. A p-type collector layer 10 is furtherformed so as to be adjacent to a lower surface of the n-type field stoplayer 9. A collector electrode 14 is connected to a lower surface of thep-type collector layer 10. It is a matter of course that thesemiconductor device may be an IGBT having no n-type field stop layer 9.

Structural parameters except the VLD region 17 are as follows. Theseparation oxide film 2 is 0.5 μm thick. The interlayer insulating film8 is 1.0 μm thick. The length Ls between the channel stopper layer 6 andthe p-type diffusion layer 17 d of the VLD region 17 as shown in FIG. 40is 140 μm. A length MFP1 of the roof of the emitter electrode 11 is 23μm. The length MFP2 in the stopper field plate 15 is 13 μm. The channelstopper layer 6 is 26 μm long. The p-type collector layer 10 has a peakconcentration of 1.8E17/cm³ and is 0.8 μm thick (diffusion depth). Then-type field stop layer 9 has a peak concentration of 2.2E15/cm³ and is28.5 μm thick. The distance (L_(sC)) between the clip p layer 17 e andthe nearest p-type diffusion layer 17 d of the VLD region 17 is about113 μm. The length MFP4 a of the active region-side roof of the clipfield plate 15 a is 12 μm, and the length MFP4 b of the chip outercircumferential side roof of the clip field plate 15 a is 6 μm.

(Operations)

Operations and effects in the basic structure of the invention will bedescribed with reference to FIGS. 2A to 2D.

FIGS. 2A to 2D are schematic views showing the basic operation of theinvention based on the sectional configuration of important part shownin FIG. 1. FIG. 2A is a sectional view of important part showing only anelement surface portion in FIG. 1. FIG. 2B is a graph of an electricfield intensity distribution along a depth-direction cut line passingthrough a point M perpendicularly from a device surface in the sectionalview shown in FIG. 2A. FIG. 2C is a graph of an electric field intensitydistribution along a cut line parallel to the device surface in a rangeof from a point A to a point D in FIG. 2A. Incidentally, the electricfield intensity distribution is a distribution in a state where avoltage equivalent to the avalanche breakdown voltage of the element isapplied between the collector electrode 14 and the emitter electrode 11under the condition that the MOS gate is off.

The structure of the semiconductor device according to the invention ischaracterized in that the clip p layer 17 e is provided on the outercircumferential side of the VLD region 17 as described above. Accordingto this structural characteristic, the depletion layer can be preventedfrom spreading to the chip outer circumferential side because ofnegative external charge 42 coming flying onto the upper surface of thetermination structure region 33.

That is, according to the structure of the invention, as shown in FIG.2C, potential is shared among three regions, that is, (1) the VLD region17 (section A-B in FIGS. 2A and 2C), (2) the vicinity of the clip player 17 e (section B-C and its vicinity in FIGS. 2A and 2C) and (3) aregion between the clop p layer 17 e and the channel stopper layer 6(section C-D in FIGS. 2A and 2C), so that the breakdown voltage, thatis, a bearable voltage can be prevented from being reduced. It may behere conceivable that the vicinity of the clip p layer 17 e indicates aregion up to the circumference of a circle, for example, with thedeepest position of the clip p layer 17 e as its center and the order ofa diffusion depth of the clip p layer 17 e from the center as itsradius. As for the vicinity of another position, a region up to thecircumference of a circle with the distance of similar order as itsradius can be assumed.

Assume now the case where external charge is sufficiently low,specifically, the case of |Qss|≦1E10/cm² in which Qss is externalcharge. This is equivalent to the electric field intensity distributionindicated by the thick solid line in FIG. 2C. In this case, thetermination structure region is formed so that the applied voltage ismainly shared to the VLD region 17 and the region near the clip p layer17 e. That is, electric field intensity becomes the highest at a point Cin front of the clip p layer 17 e and decreases in directions of fromthe point C to the active region side (point A) and the chip outercircumferential side (point D). A value obtained by integrating theelectric field intensity distribution in lateral distance is equivalentto the applied voltage.

Then, assume the case of positive external charge 41, that is, the caseof external charge of Qss>+1E10/cm². This is equivalent to the electricfield intensity distribution indicated by the thin dotted line in FIG.2C. In this case, the Si side surface layer of the SiO₂/Si interface isnegatively charged, so that the depletion layer (equipotential surface)shifts to the inner circumferential side of the chip. On the other hand,the VLD region 17 is effective in relaxing electric field intensity tospread the depletion layer to the chip outer circumferential side. Forthis reason, the depletion layer is spread to the clip p layer 17 e. Onthis occasion, a complementary high electric field intensity region isformed in the vicinity of the clip p layer 17 e (in the vicinity ofsection B-C in FIG. 2A) by the operation of the clip p layer 17 e. Thehigh electric field intensity region permits the bearable voltage to bekept without reduction.

Assume next the case of negative external charge 42, that is, the caseof external charge of Qss<−1E10/cm². This is equivalent to the electricfield intensity distribution indicated by the thick broken line in FIG.2C. In this case, the Si side surface layer of the SiO₂/Si interface ispositively charged, so that the depletion layer (equipotential surface)shifts to the outer circumferential side of the chip. For this reason,electric field intensity in the section A-B indicating the VLD region 17is further reduced, so that the VLD region 17 alone cannot bear a largevoltage. On this occasion, because the clip p layer 17 e of theinvention is provided as described above, a complementary high electricfield intensity region is formed in the vicinity of the clip p layer 17e. Moreover, because spreading of the depletion layer is stopped at anactive region side point D in front of the channel stopper layer 6, ahigh electric field intensity region is also formed in the section C-D.As described above, the applied voltage can be shared to the highelectric field intensity regions of the sections B-C and C-D, so thatthe breakdown voltage can be kept for negative external charge.

FIG. 2B schematically shows the electric field intensity distribution ofthe active portion. The electric field intensity distribution shown inFIG. 2B is a distribution along a cut line for cutting the element in adepth direction so as to pass through a point M at the left end of theelement sectional view of FIG. 2A. Let E_(max) be maximum electric fieldintensity in the active region 31 when a certain voltage (V₀) is appliedto thereby generate avalanche breakdown. Let E_(tm) be electric fieldintensity maximized in the termination structure region 33 regardless ofthe positive or negative polarity of external charge when the voltage(V₀) is applied. On this occasion, E_(max)≧E_(tm) is preferred. When avoltage substantially equal to the breakdown voltage is applied to theelement, a current due to avalanche breakdown is generated. It ispreferable that this avalanche current is not concentrated into a pointbut flows in the whole active region 31. This is because when avalanchebreakdown is generated at a point of the termination structure region 33prior to the active region 31, the avalanche current is concentratedinto the point so that the element may be broken to make it impossibleto keep the voltage in accordance with circumstances. Such breakdown ofthe element can be avoided when avalanche breakdown occurs in a widerange of the active region 31 at a lower voltage than that of thetermination structure region 33. In other words, it is preferable thatE_(max)≧E_(tm) holds when the same voltage (V₀) is applied. For example,the invention is designed so that avalanche breakdown occurs in thep-type base layer 5 broadly and uniformly formed in the active region31. In this manner, the avalanche current can flow in the whole activeregion 31 stably. As a result, concentration of the current into thetermination structure region 33 can be avoided.

As shown in FIG. 41, in the guard ring structure, the length(hereinafter referred to as edge length) occupied by the terminationstructure region 33 in the chip was generally large. It was furthernecessary to increase the number of guard rings to give likelihood toexternal charge, so that this promoted increase in edge length. In thisinvention, the clip p layer 17 e similar to the guard ring structure isintentionally inserted in the outer circumferential side of the VLDregion 17. With this configuration, potential can be rather shared tothe three regions, that is, (1) the VLD region 17, (2) the region nearthe clip p layer 17 e and (3) the region between the clip p layer 17 eand the channel stopper layer 6. As a result, charge resistance whichhas not been obtained in the background art can be obtained in spite ofa distance far shorter than that in the guard ring structure.

A preferred embodiment of the invention will be described.

FIG. 2D shows a depletion layer region near the clip p layer 17 e. FIG.2D is a schematic view showing a depletion layer end 50 added to anenlarged view of the vicinity of the point C in FIG. 2A. Assume the casewhere a voltage equivalent to the avalanche breakdown voltage of theelement is applied between the collector electrode 14 and the emitterelectrode 11 under the condition that the MOS gate is off. When thevoltage is applied, a depletion layer 53 spreads into the clip p layer17 e as shown in FIG. 2D. On this occasion, it is preferable that thedepth m (depletion layer width) of the depletion layer 53 inside theclip p layer 17 e is larger than the depth n of a non-depleted chargeneutral region 54 inside the clip p layer 17 e. The term “depth” means adistance in a direction perpendicular to the chip surface.

Assume that the concentration of the clip p layer 17 e is aconcentration so high that there is no depletion at all. Then, when thedepletion layer (equipotential surface) shifts to the outercircumferential side of the VLD region 17 because of external charge,spreading of the depletion layer is stopped by the clip p layer 17 e.For this reason, a local high electric field intensity point which isvery narrow and sharp compared with increase of electric field intensityas shown in FIG. 2C is generated in the vicinity of the clip p layer 17e, so that an avalanche current is generated to reduce the breakdownvoltage. Generation of such a pinpoint local high electric fieldintensity point in the vicinity of the clip p layer 17 e can be avoidedwhen both equipotential surface density and spatial gradient in theinside of the clip p layer 17 e are reduced. It is therefore preferablethat the depletion layer 53 is spread into the clip p layer 17 e assufficiently as possible. In the inside of the clip p layer 17 e, thedepletion layer 53 is located under the depletion layer end 50. In thevicinity of the point D, the depletion layer 53 is located in the left(active region 31 side) of the depletion layer end 50. Accordingly, itis preferable that the depth m (depletion layer width) of the depletionlayer 53 spreading into the clip p layer 17 e is larger than the depth nof the non-depleted charge neutral region 54 of the clip p layer 17 e asshown in FIG. 2D.

The reason will be described below more in detail. When the clip p layer17 e is depleted, the depletion layer 53 spreads in a wide range insidethe clip p layer 17 e except the high concentration portion of thesurface as shown in FIG. 2D. As will be described later, the voltageendured by the depletion layer 53 of the clip p layer 17 e when avoltage equivalent to the avalanche breakdown voltage is applied dependson the value of a rating voltage but is, for example, about 40-50 V.This potential difference is endured by the depletion layer 53 of theclip p layer 17 e. On the other hand, the critical integratedconcentration causing avalanche breakdown takes a constant value ofabout 1.2E12 atoms/cm² in the case of silicon regardless of the impurityconcentration. Accordingly, when the diffusion depth is not larger than10 μm on the assumption that the diffusion profile conforms to aGaussian function, the concentration of the clip p layer 17 e at thedepletion layer end 50 is the order of about 1.0E16 atoms/cm³. Here, thediffusion depth of the clip p layer 17 e is, for example, 5 μm. Thedepletion layer width m in the depth direction of the clip p layer 17 eis set to be 2.5 μm which is equal to the width n of the charge neutralregion 54 of the clip p layer 17 e. In this case, because the electricfield intensity distribution in the inside of the clip p layer 17 e canbe approximated to a triangle, the maximum value of electric fieldintensity (maximum electric field intensity) is about50(V)×2/2.5(μm)=4.0E5 V/cm when the two-dimensional curvature of thedepletion layer is not considered. In practice, because theequipotential surface in the clip p layer 17 e and its vicinity iscurved two-dimensionally in a direction parallel to the chip surface andin a depth direction, the absolute value of electric field intensityincreases more. The critical electric field intensity causing avalanchebreakdown depends on the impurity concentration of the depletion layerbut is about 3.0E5 to 4.0E5 V/cm. Accordingly, when the diffusion depthof the clip p layer 17 e is 5.0 μm, maximum electric field intensity canbe made smaller than the critical electric field intensity as long asthe depletion layer width m of the clip p layer 17 e is larger than thedepth n of the charge neutral region of the clip p layer 17 e. Moreover,because the integration concentration takes a constant value regardlessof the impurity concentration, the concentration of the clip p layer 17e at the depletion layer end 50 is the order of 1.0E16 atoms/cm³substantially regardless of the impurity concentration. That is, even invarious rating voltages, the concentration of the clip p layer 17 e atthe depletion layer end 50 is the same order. Accordingly, when thedepletion layer width m in the depth direction of the clip p layer 17 eis larger than the width n of the charge neutral region of the clip player 17 e, generation of the local high electric field intensity pointcan be suppressed because electric field intensity in the clip p layer17 e can be absorbed sufficiently.

For example, as shown in FIG. 1, the shape of the pn junction 43 of theVLD region 17 is a wavy shape on which the respective diffusion shapesof the p-type diffusion layers 17 a to 17 d are reflected. In thismanner, electric field intensity is distributed to the VLD region 17 sothat a high electric field intensity region is formed in the bottom ofeach of the p-type diffusion layers 17 a to 17 d. The VLD region 17 canendure the applied voltage higher by the high electric field intensityregion.

The p-type channel stopper layers 22 shown in FIG. 42 are not used inthe invention. When positive external charge comes flying onto thesurface of the termination structure region, an electron storage layeris induced in the SiO₂/Si interfaces of the n-type semiconductorsubstrate surrounded by the p⁺ guard rings 20 a to 20 f. This electronstorage layer is cut off by the p-type channel stopper layers 22 in FIG.42. On the other hand, in this invention, the VLD region 17 is providedbroadly. Even when positive external charge comes flying, the electronstorage layer is hardly formed in the VLD region 17. Accordingly, thep-type channel stopper layers 22 are not used in the invention.

Embodiment 1

Embodiment 1 of the invention will be described with reference to FIG.3.

FIG. 3 is a sectional view showing important part f a semiconductordevice according to Embodiment 1 of the invention. The point ofdifference from the structure as a basic configuration shown in FIG. 1lies in that a clip n layer 6 a higher in concentration than the n-typedrift layer 1 is formed in front of the active region side of the clip player 17 e.

As described above, when negative external charge reaches the uppersurface of the termination structure region, the Si side surface layerin the SiO₂/Si interfaces is positively charged. Holes are induced inthe interface portion (Si surface between B and C in FIG. 2A) of then-type drift layer 1 particularly low in concentration, so that achannel layer of holes is formed. A leakage current path is generatedbetween the emitter electrode 11 and the collector electrode 14 throughthe channel layer of holes, so that this causes a leakage current. Onthe contrary, the channel layer of holes can be cut off by the clip nlayer 6 a.

Structural parameters specific to Embodiment 1 are as follows. In FIG.3, the width of the clip n layer 6 a is about 2.3 μm, and the diffusiondepth of the clip n layer 6 a is about 3.0 μm. The surface concentrationof the clip n layer 6 a is 9.8E20/cm³.

A preferred mode of Embodiment 1 will be described below with referenceto FIG. 3.

It is preferable that the clip p layer 17 e is deeper than the clip nlayer 6 a.

When the depletion layer spreads into the termination structure region33 in a direction of from the chip inner circumferential side to thechip outer circumferential side, the depletion layer is not stopped bythe clip n layer 6 a so that the depletion layer can reach the clip player 17 e.

It is preferable that the clip n layer 6 a is adjacent to the clip player 17 e.

In this case, because the depletion layer little collides with the clipn layer 6 a but reaches the clip p layer 17 e, electric field intensityin the vicinity of the clip n layer 6 a can be relaxed.

It is preferable that a clip field plate 15 a is formed on the surfaceof the clip p layer 17 e.

In this case, surface potential of the clip p layer 17 e is fixed overthe whole of the region of the clip field plate 15 a. Accordingly,equipotential surface change due to external charge coming flying can bereduced more sufficiently.

It is preferable that the length MFP4 a of the roof extending in theclip field plate 15 a toward the chip inner circumferential side islarger than the length MFPb of the roof extending in the clip fieldplate 15 a toward the chip outer circumferential side.

Spreading of the depletion layer inside the clip p layer 17 e on thechip inner circumferential side is larger than that on the chip outercircumferential side. For this reason, electric field intensity in thevicinity of the chip inner circumferential side of the clip p layer 17 eincreases. Accordingly, when MFP4 a is made longer than MFP4 b, electricfield intensity in the vicinity of the chip inner circumferential sideof the clip p layer 17 e can be relaxed.

It is preferable that the clip n layer 6 a is covered with the clipfield plate 15 a through the separation oxide film 2 or the interlayerinsulating film 8.

The clip n layer 6 a is of an n type and higher in concentration thanthe n-type drift layer 1. For this reason, when the depletion layerreaches the clip n layer 6 a, electric field intensity increasesextremely largely. When the clip n layer 6 a is formed on the outercircumferential side of the inner circumferential side end portion ofthe clip field plate 15 a, the clip n layer 6 a is covered with the clipfield plate 15 a through the separation oxide film 2 or the interlayerinsulating film 8. For this reason, the depletion layer does not reachthe clip n layer 6 a, so that increase in electric field intensity canbe prevented.

It is preferable that the VLD end portion 44 on the outercircumferential side of the VLD region 17 is located on the outercircumferential side of an emitter end portion 45 on the chip outercircumferential side of the emitter electrode 11.

Relaxation of electric field intensity in the VLD region 17 can beobtained when the potential (equipotential surface) distribution isgenerally made uniform without provision of the field plate on the uppersurface of the VLD region 17. Accordingly, it is preferable that theemitter end portion 45 of the emitter electrode 11 is provided on thechip inner circumferential side of the VLD end portion 44 so that aportion of the upper surface of the VLD region 17 not covered with thefield plate is made broad.

It is preferable that the channel stopper layer 6 is provided with astopper field plate 15, and that there is provided a spacing region 40by which the clip field plate 15 a is separated from the stopper fieldplate 15.

When the clip field plate 15 a and the stopper field plate 15 areseparated from each other on the upper surface of the spacing region 40,the equipotential surface is distributed to the spacing region 40. As aresult, even when the depletion layer shifts to the chip outercircumferential side as described above, the voltage can be endured bythe spacing region 40.

Incidentally, a gate polysilicon runner 4 b is described in the chipouter circumferential side of the active end portion 32 in FIGS. 1 and3. The term “gate polysilicon runner 4 b” means the followingconfiguration. For example, gate polysilicon 4 a formed in each unitcell (which is a unit structure of a cycle period structure composed ofa gate, a p-type base layer and an n⁺ emitter layer) of an IGBT iscollected as one layer at an end portion of a region where the unitcells are gathered. This is the gate polysilicon runner 4 b. The gatepolysilicon runner 4 b is connected to a gate pad region which is notshown but provided for connection to a gate terminal on the chipsurface. The gate polysilicon runner 4 b is a well-known technique. Inthe description of embodiments of the invention, the gate polysiliconrunner 4 b is included in the termination structure region 33, so thatthe VLD region 17 is formed so as to be located from the lower portionof the gate polysilicon runner 4 b toward the chip outer circumferentialend.

Here, a p-type base layer 5 higher in concentration than the VLD region17 may be formed between the gate polysilicon runner 4 b and the VLDregion 17. That is, the innermost circumference of the VLD region 17 isa place where the p-type dopant concentration is the highest in the VLDregion 17. Accordingly, even when the p-type base layer 5 is formed, theinfluence on the equipotential surface distribution of the terminationstructure region 33 is little. Because the concentration is high, theinfluence of external charge on the potential distribution in thevicinity of the gate polysilicon runner 4 b is rather reduced.

A method of producing the semiconductor device according to Embodiment 1of the invention will be described next. Although reference numerals andsigns indicating parts not shown in respective drawings showing stepswill appear in the following description, the parts are the same asshown in FIG. 3.

FIGS. 4A-4B, 5A-5E, 6A-6D, 7A-7D, 8, 9A-9D, 10A-10E, 11A-11B, 12A-12B,13A-13D, 14A-14C, 15A-15C and 16 are sectional views of important partin respective steps showing the method of producing the semiconductordevice according to Embodiment 1 of the invention.

(FIG. 4A) A separation oxide film 2 about 1.2 μm thick is formed as athermal oxide film or a deposit film on an upper surface of an n-typedrift layer 1. (FIG. 4B) A photo resist is applied on the separationoxide film 2 and photolithography is executed.

(FIG. 5A) Successively, the photo resist 19 is used as it is, so thatthe separation oxide film 2 is removed by wet isotropic etching or dryanisotropic etching until the n-type drift layer 1 is revealed. (FIG.5B) Then, the photo resist 19 is removed and the wafer is washed. (FIG.5C) Successively, thermal oxidation is performed so that a screen oxidefilm 27 about 50 nm thick is formed in the surface. (FIG. 5D)Successively, a photo resist 19 is applied on the whole surface of thewafer again, and photolithography is executed. (FIG. 5E) Then, boronions are implanted with acceleration energy of 45 keV and a dosequantity of 3E12/cm². Then, the photo resist 19 is removed and the waferis washed.

(FIG. 6A) Successively, after drive-in is performed in a nitrogenatmosphere at 1100° C. for 5 hours, a thermal oxide film region 2 aabout 600 nm thick is formed in the surface of the n-type drift layer 1by pyrogenic oxidation at 1150° C. Boron is diffused by the thermalbudgets, so that p-type diffusion layers 17 a to 17 d and a clip p layer17 e in the VLD region 17 are formed. In the case of the aforementioneddose quantity (3E12/cm²), the surface concentration of the clip p layer17 e is about 1E16/cm³ and the junction depth of the clip p layer 17 eis about 5-6 μm in this stage. As a result, the depth of the depletionlayer spreading into the clip p layer 17 e when, for example, a voltagesubstantially equal to the breakdown voltage is applied to the elementshown in FIG. 1 is as follows. When the surface concentration andjunction depth of the clip p layer 17 e are as described above, thedepth m of the depletion layer spreading inside the clip p layer 17 e asshown in FIG. 2D is about 4 μm. That is, the depth n of the chargeneutral region (i.e. non-depleted region) remaining in the clip p layer17 e from the element surface is about 1-2 μm. Accordingly, the width mof the depletion layer spreading inside the clip p layer 17 e is largerthan the depth n of the charge neutral region.

(FIG. 6B) Then, a photo resist 19 is applied on the whole surface of thewafer and photolithography is performed so that the resist forselectively etching a thermal oxide film region 2 a as a trench etchingmask is patterned to form a resist opening portion 47 a. (FIG. 6C)Successively, oxide film anisotropic etching is performed to remove theoxide film from the resist opening portion 47 a. (FIG. 6D) Then, thephoto resist 19 is removed.

(FIG. 7A) Silicon anisotropic etching is performed with the separationoxide film 2 and the thermal oxide film region 2 a as a mask to therebyform a trench 52. Although the depth of the trench depends on devicedesign, the depth in this embodiment is 5 μm. Isotropic chemical dryetching (CDE) may be further performed. (FIG. 7B) After the wafer iswashed, sacrificial oxidation for removing trench etching damage isperformed to form a sacrificial oxide film 2 b. (FIG. 7C) Successively,the sacrificial oxide film 2 b is removed by oxide film etching and thewafer is washed. (FIG. 7D) Successively, a gate oxide film 3 is formedby thermal oxidation or deposition or by combination of thermaloxidation and deposition. Although the thickness of the gate oxide film3 depends on device design, the thickness of the gate oxide film 3 inthis embodiment is 100 nm.

(FIG. 8) Doped polysilicon is deposited on the wafer surface so that thetrench is fully filled with the doped polysilicon. Then,photolithography is performed and the polysilicon is etched byanisotropic or isotropic dry etching to form gate polysilicon 4 a of atrench gate and a gate polysilicon runner 4 b. Then, the photo resist 19is removed and the wafer is washed.

(FIG. 9A) Then, a photo resist 19 is applied on the whole surface of thewafer and photolithography is performed to form resist opening portions47 b. (FIG. 9B) Then, boron ion implantation is performed with the photoresist 19 as a mask. Although the condition for ion implantation dependson device design, for example, the condition of 100 keV and 2.5E13/cm²is used. (FIG. 9C) Then, the photo resist 19 is removed and the wafer iswashed. (FIG. 9D) Here, the oxide film thickness of the wafer surface isreduced by wet etching to make it easy to form a clip n layer 6 a, an n⁺emitter layer 6 b and contact p⁺ regions 7 and 7 a to 7 d in thefollowing steps.

(FIG. 10A) Successively, diffusion of boron ions implanted in FIG. 9B isperformed in an inert atmosphere to form a p-type base layer 5. Althoughthe thermal budget necessary for diffusion of the p-type base layer 5depends on device design, for example, 1100° C., 220 minutes and anitrogen atmosphere are used. (FIG. 10B) Successively, a photo resist isapplied on the whole surface of the wafer and photolithography isperformed so that the resist is patterned to form a resist openingportion 47 c. (FIG. 10C) Then, arsenic ion implantation is performed.Although the condition for arsenic ion implantation depends on devicedesign, for example, the condition for arsenic ion implantation is setat 20 keV and 4-5E15/cm². (FIG. 10D) Then, the photo resist 19 isremoved and the wafer is washed. (FIG. 10E) Then, arsenic ions implantedin FIG. 10C are activated by heat treatment to thereby form an n⁺emitter layer 6 b, a channel stopper layer 6 and a clip n layer 6 a. Asan example of a thermal budget for the heat treatment, a nitrogenatmosphere, 1100° C. and 30 minutes are used.

(FIG. 11A) Photolithography is performed so that a photo resist 19 ispatterned to form resist opening portions 47. (FIG. 11B) Successively,boron ion implantation is performed. Although the condition for boronion implantation depends on device design, for example, the conditionfor boron ion implantation is set at 150 keV and 2-3E15/cm².

(FIG. 12A) Successively, the photo resist is removed and the wafer iswashed. (FIG. 12B) Then, boron implanted in FIG. 11B is activated, forexample, by heat treatment at 970° C. for 30 minutes to thereby form p⁺regions 7, 7 a, 7 b and 7 d.

(FIG. 13A) An interlayer insulating film 8 is deposited on the uppersurface of the wafer. For example, the interlayer insulating film 8 isprovided as a composite film formed from a chemical vapor deposition(CVD) high temperature oxide (HTO) film and a boron phosphorus silicateglass (BPSG) film. As an example of the thickness of the interlayerinsulating film 8, for example, the HTO is 200 nm thick and the BPSG is1000 nm thick. Successively, the composite film is annealed at 970° C.for 20 minutes in a nitrogen atmosphere. (FIG. 13B) Photolithography isperformed so that a photo resist 19 is patterned to form resist openingportions 47 d. (FIG. 13C) Successively, the interlayer insulating film 8in the resist opening portions 47 d is etched by anisotropic etching tothereby open contact portions between an electrode which will be formedlater and silicon as a base material. (FIG. 13D) Then, the photo resist19 is removed and the wafer is washed.

(FIG. 14A) A back-grinding tape 18 is stuck to the front surface of thewafer and chemical mechanical polishing (CMP) 48 is applied to the rearsurface of the wafer to make the wafer thin. (FIG. 14B) Successively,n-type dopant ions (phosphorus, selenium, hydrogen, etc.) for forming ann-type field stop layer 9 are implanted in the rear surface of thewafer. The dose and acceleration energy of ion implantation are selectedsuitably. Successively, boron ion implantation is performed for forminga p-type collector layer 10. The dose and acceleration energy of boronion implantation are selected suitably. (FIG. 14C) Then, theback-grinding tape 18 is removed and the n-type field stop layer 9 andthe p-type collector layer 10 are activated by thermal driving or laserannealing. In the case of thermal driving, 950° C., 30 minutes and anitrogen atmosphere are used as an example of a thermal budget.

Successively, though not shown, the wafer is washed and a natural oxidefilm on the contact bottom surface is removed by wet etching. (FIG. 15A)Then, a barrier metal (such as Ti/TiN 200 nm/500 nm) is sputtered andthen an Al—Si electrode material is sputtered onto the front surface ofthe wafer to thereby form a metal film 49 for emitter electrode 11.Incidentally, the barrier metal may be dispensed with. Though not shown,after formation of the barrier metal, in accordance with necessity, atungsten film may be deposited by a CVD method and a tungsten plug maybe formed in a contact hole by etching-back or CMP before the Al—Sielectrode material is sputtered to form an electrode layer on the frontsurface of the wafer. (FIG. 15B) Successively, photolithography isperformed so that a photo resist 19 is patterned to form resist openingportions 47 e. (FIG. 15C) Successively, etching is performed so that themetal film 49 in the resist opening portions 47 e is removed to form anemitter electrode 11, a clip field plate 15 a and a stopper field plate15. Then, the photo resist 19 is removed.

(FIG. 16) A silicon nitride film having a composition close to Si₃N₄ isformed on the front surface of the wafer so that the film is provided asa passivation film 13 about 300 nm thick. Or a 28 μm-thick polyimidefilm for suppressing degradation due to cosmic radiation may be formedseparately from the silicon nitride film. Pads for the emitter electrode11 and a gate electrode not shown are formed via photolithography and anetching process. Finally, an Al—Si electrode material and Ti/Ni/Au, etc.necessary for good soldering to a package are sputtered onto the rearsurface of the wafer to thereby form a collector electrode 14. Thus, atermination structure shown in FIG. 3 is completed.

Incidentally, it is preferable that the dose quantity of boron ionimplantation for forming the VLD region in FIGS. 5A to 5E is smallerthan the dose quantity of boron ion implantation for forming the p-typebase layer 5 in FIGS. 9A to 9D. If the dose quantity for the VLD regionis larger than the dose quantity for the p-type base layer 5, the higherconcentration of the VLD region 17 makes spreading of the depletionlayer hard when the depletion layer spreads in the VLD region 17 fromthe p-type base layer 5 toward the chip outer circumferential side. As aresult, potential sharing to the three regions ((1) the VLD region 17,(2) the vicinity of the clip p layer 17 e and (3) the region between theclip p layer 17 e and the channel stopper layer 6) which is theoperation of the invention cannot be performed. Because the depletionlayer need spread easily toward the outer circumferential side in theVLD region, it is preferable that the dose quantity of boron in the VLDregion is smaller than the dose quantity of boron in the p-type baselayer 5. When the dose quantity of boron in the VLD region is set to besmaller than the dose quantity of boron in the p-type base layer 5, itis a matter of course that the concentration of the p-type base layer 5high in acceptor concentration becomes higher than that of the VLDregion. As a result, avalanche breakdown occurs in the p-type base layer5 at a voltage lower than that of the VLD region. Consequently, asdescribed above with reference to FIGS. 2A to 2D, an avalanche currentflows in the active region 31. Accordingly, current concentration intothe termination structure region 33 can be avoided.

A modification of the producing method according to Embodiment 1 of theinvention will be described below.

FIGS. 17A to 17C, FIGS. 18A to 18C, FIGS. 19A to 19C, FIGS. 20A to 20Cand FIG. 21 are sectional views of important part in respective stepsconcerned with the modification of the producing method according toEmbodiment 1 of the invention. In the process shown in FIGS. 4A and 4Band FIGS. 5A to 5E, boron ion implantation for forming the VLD region 17is performed once. The point of difference is in that boron ionimplantation is divided into a plurality of times in this modification.First, steps from FIG. 4A to FIG. 5C are executed.

(FIG. 17A) Successively, a photo resist 19 is applied on the wholesurface of the wafer again and photolithography is performed. On thisoccasion, the photo resist 19 is patterned so that only regions wherethe p-type diffusion layer 17 a and the clip p layer 17 e in FIG. 3 willbe formed are opened. (FIG. 17B) Then, boron ions are implanted at 45keV and 1E12/cm². In the boron ion implantation, ions are implanted inthe portion of the p-type diffusion layer 17 a in FIG. 3. (FIG. 17C)Then, the photo resist 19 is removed and the wafer is washed.

(FIG. 18A) Successively, a photo resist 19 is applied on the wholesurface of the wafer and photolithography is performed. On thisoccasion, the photo resist 19 is patterned to form an ion implantationopening portion 51 a so that only a region where the p-type diffusionlayer 17 b in FIG. 3 will be formed is opened. (FIG. 18B) Then, boronions are implanted at 45 keV and 1E12/cm². In the boron ionimplantation, ions are implanted into the portion of the p-typediffusion layer 17 b in FIG. 3. (FIG. 18C) Then, the photo resist 19 isremoved and the wafer is washed.

(FIG. 19A) Successively, a photo resist 19 is applied on the wholesurface of the wafer again and photolithography is performed. On thisoccasion, the photo resist 19 is patterned to form an ion implantationopening portion 51 b so that only a region where the p-type diffusionlayer 17 c in FIG. 3 will be formed is opened. (FIG. 19B) Then, boronions are implanted at 45 keV and 0.5E12/cm². In the boron ionimplantation, ions are implanted into the portion of the p-typediffusion layer 17 c in FIG. 3. (FIG. 19C) Then, the photo resist 19 isremoved and the wafer is washed.

(FIG. 20A) Successively, a photo resist 19 is applied on the wholesurface of the wafer again and photolithography is performed. On thisoccasion, the photo resist 19 is patterned to form an ion implantationopening portion 51 c so that a region where the p-type diffusion layer17 d in FIG. 3 will be formed is opened. (FIG. 20B) Then, boron ions areimplanted at 45 keV and 0.5E12/cm². In the boron ion implantation, ionsare implanted into the portion of the p-type diffusion layer 17 d inFIG. 3. (FIG. 20C) Then, the photo resist 19 is removed and the wafer iswashed.

(FIG. 21) Successively, after drive-in is performed at 1100° C. for 5hours in a nitrogen atmosphere, a thermal oxide film region 2 a about600 nm thick is formed in the front surface of the n-type drift layer 1by well-known pyrogenic oxidation at 1150° C. By the thermal budget ofdrive-in and pyrogenic oxidation, the p-type diffusion layers 17 a to 17d and the clip p layer 17 e in the VLD region are formed. The producingprocess after this step may go through the same procedure as on andafter FIG. 6B.

In this modification, boron ion implantation is divided into a pluralityof times (four times). For this reason, exactly, the averageconcentration distribution of the VLD region is slightly different froma processing result shown in FIGS. 4A-4B, 5A-5E, 6A-6D, 7A-7D, 8, 9A-9D,10A-10E, 11A-11B, 12A-12B, 13A-13D, 14A-14C, 15A-15C and 16 but thetotal dose quantity and the function of electric field relaxation andpotential sharing are equal. The number of times for ion implantationneed not be limited to four in this embodiment. As described above, whenboron ion implantation is divided into a plurality of times for formingthe VLD region, the number of masking steps increases but the functionof electric field relaxation and potential sharing can be tuned finely.

FIG. 22 is a graph showing surface charge dependence of breakdownvoltage in Embodiment 1 and the background art. In the structure of thebackground art in which only the VLD region is formed, it is found thatthe breakdown voltage is reduced to 1200V or lower when the negativecharge is particularly −0.9E12/cm² or lower. This is because, in thebackground art, the depletion layer punches through the channel stopperlayer and shifts to the chip outer circumferential side so that thetermination structure region 33 cannot breakdown the voltage when Qss isnot higher than −0.9E12/cm².

On the other hand, in Embodiment 1, the breakdown voltage can be keptnot lower than 1300V when Qss is not higher than −1.2E12/cm².

FIG. 23 shows an electric field intensity distribution in an Si surfaceof an SiO₂/Si interface at device breakdown (avalanche breakdownvoltage) in each of Embodiment 1 and the background art. Qss is−1.0E12/cm². In the background art, electric field intensity ismaximized on a side near the inside of the channel stopper layer 6 anddecreases toward the VLD region. On the other hand, in Embodiment 1,electric field intensity increases in a section ranging from the clip player 17 e to the VLD region. Moreover, the distribution is uniform withrespect to lateral distance. In addition, though the depletion layer isapt to shift to the chip outer circumferential side because of negativeexternal charge, electric field intensity in the VLD region is higherthan that in the background art. That is, as described above, potentialis shared to the three regions, that is, (1) the VLD region 17, (2) thevicinity of the clip p layer 17 e and (3) the region between the clip player 17 e and the channel stopper layer 6 as a result of formation ofthe clip p layer 17 e.

Embodiment 2

A semiconductor device according to Embodiment 2 of the invention willbe described with reference to FIG. 24.

FIG. 24 is a sectional view showing important part of a semiconductordevice according to Embodiment 2 of the invention. The point ofdifference of Embodiment 2 from Embodiment 1 (FIG. 3) lies in that asurface p-type field layer 16 a higher in concentration than the n-typedrift layer 1 is provided on the chip outer circumferential side of thep-type diffusion layer 17 d in the VLD region 17. The surface p-typefield layer 16 a is separated from the p-type diffusion layer 17 d andthe clip p later 17 e or the clip n layer 6 a.

The length L of the surface p-type field layer 16 a in a direction alongthe SiO₂/Si interface is 30 μm. The dose quantity of the surface p-typefield layer 16 a is 5.4E11/cm². When the concentration of the n-typedrift layer 1 is 9E13/cm³, the dose quantity of the surface p-type fieldlayer 16 a is preferably in a range of from 5 to 8E11/cm². The surfacep-type field layer 16 a is about 9.3 μm away from the p-type diffusionlayer 17 d.

When external charge is negative, the Si surface of the terminationstructure region 33 is positively charged and the depletion layer shiftsto the chip outer circumferential side. Particularly when theconcentration of the n-type drift layer 1 is low (for example, nothigher than 6E13/cm³), the absolute value of spatial charge density inthe surface of the n-type drift layer 1 in the termination structureregion 33 decreases more remarkably so that the depletion layer spreadseasily. For this reason, the depletion layer is apt to punch through then-type channel stopper layer 6 beyond the clip p layer 17 e. By punchingthrough, an electron storage layer channel is formed in the SiO₂/Siinterface. As a result, the leakage current increases or the breakdownvoltage decreases in accordance with change in spatial charge density.Accordingly, the surface p-type field layer 16 a higher in concentrationthan the n-type drift layer 1 is formed to thereby suppress spreading ofthe depletion layer and exert an effect of cutting off the electronstorage layer channel.

Embodiment 3

A semiconductor device according to Embodiment 3 of the invention willbe described with reference to FIG. 25.

FIG. 25 is a sectional view showing important part of a semiconductordevice according to Embodiment 3 of the invention. The point ofdifference of Embodiment 3 from Embodiment 2 (FIG. 24) lies in that theposition of the peak concentration of the surface p-type field layer 16a in FIG. 24 is shifted from the chip surface to the inside of then-type drift layer 1 so that the surface p-type field layer 16 a ischanged to a so-called embedded layer. The embedded layer is calledembedded p-type field layer 16 b. The embedded p-type field layer 16 bis separated from the p-type diffusion layer 17 d and the clip p layer17 e or the clip n layer 6 a. FIG. 29 is a graph showing a dopingconcentration distribution in a depth direction of each of the surfacep-type field layer 16 a in Embodiment 2 and the embedded p-type fieldlayer 16 b in Embodiment 3. As for Embodiment 3, the structure in whichthe peak concentration of the embedded p-type field layer 16 b is7.5E15/cm³ is referred to as “structure 3-a”. The dose quantity of theembedded p-type field layer 16 b in the structure 3-a is 7.3E11/cm². Onthe other hand, the structure in which the peak concentration of theembedded p-type field layer 16 b is 1E16/cm³ is referred to as“structure 3-b”. The dose quantity of the embedded p-type field layer 16b in the structure 3-b is 9.8E11/cm². The embedded p-type field layer 16b in either the structure 3-a or the structure 3-b is about 9.3 μm awayfrom the p-type diffusion layer 17 d.

The embedded p-type field layer 16 b in the structure 3-a has a largeeffect on suppressing spreading of the depletion layer for negativeexternal charge compared with the surface p-type field layer 16 a inEmbodiment 2. That is, as for the depletion layer spreading laterally inthe vicinity of the surface of the termination structure region from theactive region side, the equipotential surface is curved more largely asthe highest concentration position becomes deeper from the surface.Because electric field intensity is proportional to spatial gradient ofequipotential surface and conforms to the Poisson's equation (divE=ρ inwhich E is electric field intensity, and ρ is charge density), electricfield intensity in the vicinity of the embedded p-type field layer 16 bincreases so that a larger voltage can be withstood.

A method of producing the semiconductor device according to Embodiment 2or 3 of the invention will be described next. The producing method inEmbodiment 2 is mainly classified into two. One is a method in which aresist opening portion 35 a is also formed in a region where the surfacep-type field layer 16 a will be formed, so that boron ions are implantedby the VLD region ion implantation process in Embodiment 1, as shown inFIG. 26. By this method, the surface p-type field layer 16 a with thesame concentration distribution as that of the VLD region is formed. Theother method is as follows. First, as shown in FIG. 27A, after formationof the contact p⁺ region 7, etc., a resist is applied andphotolithography is performed to form an opening portion 35 b in whichthe resist is opened only in a region where the surface p-type fieldlayer 16 a will be formed. Successively, as shown in FIG. 27B, theseparation oxide film 2 is opened by etching. Then, as shown in FIG.27C, boron ion implantation is performed additionally and then the photoresist 19 is removed. Successively, as shown in FIG. 27D, heat treatmentat 950° C. is performed for about 30 minutes to form the surface p-typefield layer 16 a. The method of producing either the structure 3-a orthe structure 3-b is the same as the producing method in Embodiment 2.Particularly in the case of the embedded p-type field layer 16 b, thesecond method can form the embedded p-type field layer 16 b singlybecause the position of the peak concentration of the embedded p-typefield layer 16 b is deep. For example, when acceleration energy of boronion implantation is in a range of from 100 keV to 2.3 MeV, bothinclusively, the embedded layer can be formed.

FIG. 28 is a graph showing surface charge dependence of breakdownvoltage in each of Embodiment 2, Embodiment 3 (structure 3-a andstructure 3-b) and Embodiment 1. In Embodiment 1, surface chargedependence of breakdown voltage is improved compared with the backgroundart as shown in FIG. 22 but the breakdown voltage is reduced to 1100V at−0.5E12/cm². On the other hand, when the surface p-type field layer 16 ais provided as in Embodiment 2, a breakdown voltage of 1200V or higheris exhibited at the same charge as a result of the aforementionedoperation. In the case of the embedded p-type field layer 16 b, thebreakdown voltage decreases at −0.8E12/cm² or lower when the dosequantity is 7.3E11/cm², but reduction in breakdown voltage can besuppressed when the dose quantity is increased to 9.8E11/cm².

FIGS. 30A and 30B show electrostatic potential (equipotential surface)distributions at avalanche breakdown in Embodiment 2 and structure 3-a.FIG. 30A shows Embodiment 2, and FIG. 30B shows structure 3-a. Qss is−7.5E11/cm². The equipotential surface interval is 43.3V. The differencein equipotential surface distribution between Embodiment 2 and structure3-a is observed in the vicinity of the p-type field layer in the surface(upper side in FIGS. 30A and 30B) enclosed by the solid broken line.That is, in the structure 3-a, it is found that the equipotentialsurface is curved at a deeper position. This curve permits electricfield intensity to increase and a higher voltage to be withstood.

Incidentally, in either of Embodiment 2 and structure 3-a, it is foundthat the clip p layer is sandwiched between two equipotential surfaces.That is, the voltage shared to the clip p layer is substantially equalto the equipotential surface interval, that is, in a range of from 40 to50V.

Embodiment 4

A semiconductor device according to Embodiment 4 of the invention willbe described with reference to FIG. 31.

FIG. 31 is a sectional view showing important part of a semiconductordevice according to Embodiment 4 of the invention. The difference ofEmbodiment 4 from Embodiment 1 (FIG. 3) is as follows. The separationoxide film 2 and the interlayer insulating film 8 formed on the surfaceof the p-type diffusion layer 17 b are partially opened in the depthdirection of the paper surface (hereinafter referred to as lengthwisedirection) to form opening portions 35 c. A VLD field plate 11 a isnewly provided together with a contact p⁺ region 7 c in each openingportion 35 c. The length of a roof of a chip outer circumferential sideof the VLD field plate 11 a is MF3 b.

It is preferable that the distance between opening portions 35 cadjacent to each other in the lengthwise direction is larger than 16times the width of the opening portion 35 c in a direction of from thechip inner circumferential side to the chip outer circumferential side.For example, when the opening portion 35 c is 6 μm long, the distancebetween opening portions 35 c adjacent in the lengthwise direction isnot smaller than 100 μm. When the SiO₂/Si interface is positivelycharged because of negative external charge coming flying, the depletionlayer spreading to the VLD region 17 extends to the p⁺ region 7 c. Onthis occasion, extension of the depletion layer is suppressed in theplace of the p⁺ region 7 c. Accordingly, if the p⁺ region 7 c is presentin the all region in the lengthwise direction, a unique high electricfield intensity region is generated in the vicinity of the p⁺ region 7 cso that the breakdown voltage is reduced. Therefore, when the p⁺ region7 c is disposed partially, extension of the depletion layer is kept inthe place surrounded by the p⁺ regions 7 c adjacent in the lengthwisedirection. Accordingly, reduction in breakdown voltage can besuppressed.

This embodiment is further characterized in that the VLD field plates 11a connected to the p⁺ regions 7 c respectively are partially scatteredon the interlayer insulating film 8 between the p⁺ regions 7 c adjacentto one another in the lengthwise direction so as to correspond to the p⁺regions 7 c. By the VLD field plates 11 a, electric field intensitybetween the p⁺ region 7 c and the p-type diffusion layer 17 c or then-type drift layer 1 slightly increases in the surface of the chip sothat the breakdown voltage increases. The length of the VLD region 17can be shortened in accordance with the improved value of the breakdownvoltage. That is, the object of partially forming the VLD field plates11 a is to slightly increase electric field intensity in the inside ofthe VLD region 17 for positive external charge coming flying and relaxelectric field intensity to the chip outer circumferential side tosuppress too strong increase of electric field intensity to therebyimprove the breakdown voltage and shorten the length of the VLD region17.

The process flow of the device according to this embodiment is the sameas the process flow in Embodiment 1. The p⁺ regions 7 c are formedsimultaneously with the formation of the p⁺ regions 7, 7 a and 7 b.Contacts with the p⁺ regions 7 c are formed simultaneously with theformation of contacts with the interlayer insulating film 8. The VLDfield plates 11 a are formed simultaneously with the emitter electrode11, the stopper field plate 15 and the clip field plate 15 a.

FIG. 32 is a graph showing surface charge dependence of breakdownvoltage in Embodiment 4. The concentration of the n-type drift layer 1is 6E13/cm³. In a legend of FIG. 32, “D8C4B8” shows that eight p-typediffusion layers 17 d, four p-type diffusion layers 17 c and eightp-type diffusion layers 17 b are provided in the VLD region. Otherparameters except addition of VLD field plates 11 a and p⁺ regions 7 care the same as in Embodiment 1. In the legend, “D8C4B6” shows thateight p-type diffusion layers 17 d, four p-type diffusion layers 17 cand six p-type diffusion layers 17 b less than D8C4B8 by two areprovided in the VLD region. The configuration of the VLD region andother configurations except addition of VLD field plates 11 a and p⁺regions 7 c are the same as in Embodiment 1. With respect to the lengthof each VLD field plate 11 a, the distance from the center of theseparation oxide film 2 adjacent to the p⁺ region 7 c on the chip outercircumferential side of the p⁺ region 7 c to the chip outercircumferential side end portion of the VLD field plate 11 a is regardedas MFP3 b. The value of MFP3 b is set to be 5 μm.

From FIG. 32, when positive external charge is 1E12/cm2, the provisionof the VLD field plates 11 a and the p⁺ regions 7 c permits thebreakdown voltage of the element of D8C4B8 to increase by 58V and thebreakdown voltage of the element of D8C4B6 to increase by 160V.

The reason why increase in breakdown voltage of the element of D8C4B6having the VLD field plates 11 a and the p⁺ regions 7 c is large is asfollows. In D8C4B8, six p-type diffusion layers 17 b are disposedbetween the p⁺ region 7 c and the p⁺ region 7 b. Here, the distance fromthe chip outer circumferential side end portion of the p-type diffusionlayer 17 c in the chip surface to the chip outer circumferential sideend portion of the p-type diffusion layer 17 d is 93.3 μm. On the otherhand, in D8C4B6 having the VLD field plates 11 a and the p⁺ regions 7 c,four p-type diffusion layers 17 b are disposed between the p⁺ region 7 cand the p⁺ region 7 b. The distance from the chip outer circumferentialside end portion of the p⁺ regions 7 c in the chip surface to the chipouter circumferential side end portion of the p-type diffusion layer 17d is 93.3 μm. That is, in the element having VLD field plates 11 a andp⁺ regions 7 c, the breakdown voltage of the element of D8C4B6 becomesclose to the breakdown voltage of the element of D8C4B8 because of theaforementioned operation and effect. As a result, the breakdown voltagecan be kept even when the number of p-type diffusion layers 17 b isreduced by two, so that the length of the termination structure regioncan be shortened by about 25 μm.

Embodiment 5

A semiconductor device according to Embodiment 5 of the invention willbe described with reference to FIG. 33.

FIG. 33 is a sectional view showing important part of a semiconductordevice according to Embodiment 5 of the invention. Embodiment 5 ischaracterized in that VLD field plates 11 a shown in Embodiment 4 (FIG.31) are provided in Embodiment 3 (FIG. 25) having the embedded p-typefield layer 16 b. Incidentally, it is a matter of course that VLD fieldplates 11 a may be provided not in Embodiment 3 but in Embodiment 2(FIG. 24) having the surface p-type field layer 16 a. When the processflow in Embodiment 2 or 3 is incorporated in the process flow inEmbodiment 4, production in Embodiment 5 can be performed easily.

FIG. 34 is a graph showing surface charge dependence of breakdownvoltage in Embodiment 5. The concentration of the n-type drift layer 1is 9E13 cm⁻³. Expressions “D8C4B8” and “D8C4B6” in the legend are thesame as in Embodiment 4.

From FIG. 34, when positive external charge is 1E12 cm⁻², the provisionof the VLD field plates 11 a and the p⁺ regions 7 c permits thebreakdown voltage of the element of D8C4B8 to increase by 58V and thebreakdown voltage of the element of D8C4B6 to increase by 55V.

The reason why increase in breakdown voltage of the element of D8C4B6having the VLD field plates 11 a and the p⁺ regions 7 c is large is asfollows. In D8C4B8, six p-type diffusion layers 17 b are disposedbetween the p⁺ region 7 c and the p⁺ region 7 b. On the other hand, inD8C4B6 having the VLD field plates 11 a and the p⁺ regions 7 c, fourp-type diffusion layers 17 b are disposed between the p⁺ region 7 c andthe p⁺ region 7 b. The distance from the chip outer circumferential sideend portion of the p⁺ regions 7 c in the chip surface to the chip outercircumferential side end portion of the p-type diffusion layer 17 d is93.3 μm in either case.

That is, in the element having VLD field plates 11 a and p⁺ regions 7 c,the breakdown voltage of the element of D8C4B6 becomes close to thebreakdown voltage of the element of D8C4B8 because of the aforementionedoperation and effect. As a result, the breakdown voltage can be kepteven when the number of p-type diffusion layers 17 b is reduced by two,so that the length of the termination structure region can be shortenedby about 25 μm.

Embodiment 6

A semiconductor device according to Embodiment 6 of the invention willbe described with reference to FIG. 35.

FIG. 35 is a sectional view showing important part of a semiconductordevice according to Embodiment 6 of the invention. The difference ofEmbodiment 6 from Embodiment 1 (FIG. 3) lies in that pn junctionsurfaces of the p-type diffusion layers 17 a to 17 d in the VLD region17 are not wavy as shown in Embodiment 1 but overlap one another broadlyto form an envelope curve 28. As for the envelope curve 28, photo resistopening portions at ion implantation may be made narrower than the widthof opening portions in Embodiment 1, etc. or the thermal budget ofthermal diffusion may be made so high that overlapping of the lateraldiffusion portions 34 adjacent to one another (in FIG. 1) is widened. Asfor the thermal budget, in the well known method, for example, themaximum temperature may be increased by 50-100° C. or the diffusing timemay be increased by several hours. Or as will be described later, thisembodiment can be also obtained by a method in which boron ionimplantation for the VLD region 17 is performed via an oxide film havinga tapered thickness (slope shape like an inclination of a mountain).

In this manner, the acceptor concentration distribution is smoothenedmore sufficiently so that the depletion layer is apt to spread into theVLD region more remarkably.

Embodiment 7

A semiconductor device according to Embodiment 7 of the invention willbe described with reference to FIGS. 36 and 37.

FIGS. 36 and 37 are sectional views of important part showing a processof producing a semiconductor device according to Embodiment 7 of theinvention. The difference of Embodiment 7 from Embodiment 1 lies in thata thickness distribution is given to the screen oxide film 27 in FIG.5C. That is, for example, assume that the highest concentration portionsin the VLD region 17 in FIG. 35 are p-type diffusion layers 17 a and 17b. Assume that the thickness of a screen oxide film 27 a in a region forforming the p-type diffusion layers 17 a and 17 b is the same as inEmbodiment 1. Next, the thickness of a screen oxide film 27 b in aportion for forming the p-type diffusion layer 17 c high inconcentration is made larger. The thickness of a screen oxide film 27 cin a portion for forming the p-type diffusion layer 17 d lowest inconcentration is made largest. In this manner, screen oxide films 27 ato 27 c having thicknesses as shown in FIG. 36 are formed. Boron ionsare implanted as shown in FIG. 36. On this occasion, as the thickness ofthe screen oxide film increases, the range of implanted boron ions fromthe silicon surface becomes shallower. A VLD region 17 composed ofp-type diffusion layers 17 a to 17 d and a clip p layer 17 e as shown inFIG. 37 are then formed by a drive-in process.

Embodiment 8

A semiconductor device according to Embodiment 8 of the invention willbe described with reference to FIGS. 38A to 38E and FIGS. 39A to 39D.

FIGS. 38A to 38E and FIGS. 39A to 39D are sectional views of importantpart showing a process of producing a semiconductor device according toEmbodiment 8 of the invention. The difference of Embodiment 8 fromEmbodiment 1 lies in that a thickness distribution is given to theseparation oxide film 2 in FIGS. 5A to 5E.

(FIG. 38A) Photolithography is applied to an upper surface of aseparation oxide film 2 formed on a surface of an n-type drift layer 1by thermal oxidation. (FIG. 38B) While a photo resist 19 is used as amask, arsenic ions for damaging the surface of the separation oxide film2 are implanted in a region where the VLD region will be formed. Thecondition for arsenic ion implantation is, for example, a dose quantityof 1E15 atoms/cm² and acceleration energy of 40 keV. Of course, therelation between acceleration energy and the thickness of the separationoxide film 2 is based on the assumption that implanted arsenic ions donot reach the n-type drift layer 1 but the range of arsenic ions reachesthe surface of the separation oxide film 2. (FIG. 38C) Successively, thephoto resist 19 is removed and a photo resist 19 is applied on the uppersurface of the separation oxide film 2 again and patterned byphotolithography. (FIG. 38D) Successively, while the photo resist 19 isused as a mask, the separation oxide film 2 is etched by wet etching. Onthis occasion, the etching rate in the upper surface of the separationoxide film 2 becomes very large only in the region damaged in FIG. 38B.For this reason, the surface of the separation oxide film 2 is taperedso that a tapered oxide film 37 is formed. (FIG. 38E) Then, the photoresist 19 is removed and the wafer is washed.

(FIG. 39A) Successively, a screen oxide film 27 is formed to have athickness of 500 nm. (FIG. 39B) Then, a photo resist 19 is appliedagain. (FIG. 39C) Successively, the photo resist is patterned byphotolithography and boron ions are implanted. The condition for boronion implantation is, for example, acceleration energy of 45 keV and adose quantity of 3E12/cm². (FIG. 39D) Then, after the photo resist 19 isremoved, the wafer is washed and p-type diffusion layers 17 a to 17 dare formed via drive-in. The pn junction surfaces of the p-typediffusion layers 17 a to 17 d form an envelope curve 28 by reflectingthe shape of the tapered oxide film.

What is claimed is:
 1. A semiconductor device comprising: a firstelectrode formed on a first principal surface of a first conductivitytype semiconductor substrate; a second electrode formed on a secondprincipal surface of the semiconductor substrate; a base layer of asecond conductivity type formed on the first principal surface of thesemiconductor substrate and connected to the first electrode; a VLDregion of the second conductivity type provided on an outercircumferential side of the base layer; a floating region of the secondconductivity type provided on an outer circumferential side of the VLDregion and separated from the VLD region; and a stopper layer of thefirst or second conductivity type provided on an outer circumferentialside of the VLD region and separated from the VLD region, wherein thefloating region further comprises: a field region of the secondconductivity type provided on the outer circumferential side of the VLDregion and separated from the VLD region and which is lower in impurityconcentration than the VLD region, and a first clip layer of the secondconductivity type which is provided between the field region and thestopper layer and separated from the field region and the stopper layerand which is higher in impurity concentration than the field region. 2.The semiconductor device according to claim 1, wherein the first cliplayer is embedded in the semiconductor substrate.
 3. The semiconductordevice according to claim 1, wherein an electrostatic potential isshared among the VLD region, in the vicinity of the first clip layer anda region between the first clip layer and the stopper layer by forming acomplementary high electric field intensity region in the vicinity ofthe first clip layer.
 4. The semiconductor device according to claim 1,wherein a depth of the field region is shallower than a depth of thefirst clip region.
 5. The semiconductor device according to claim 1,further comprising a second clip layer of the first conductivity typewhich is provided on the outer circumferential side of the VLD regionand on an inner circumferential side of the first clip layer so as to beseparated from the VLD region and which is higher in concentration thanthe semiconductor substrate.
 6. The semiconductor device according toclaim 5, wherein a depth of the second clip layer is shallower than adepth of the first clip layer.
 7. The semiconductor device according toclaim 5, wherein the second clip layer is adjacent to the first cliplayer.
 8. The semiconductor device according to claim 1, furthercomprising a first field plate formed on a surface of the first cliplayer.
 9. The semiconductor device according to claim 8, wherein alength of the first field plate toward an inner circumferential side ofthe first clip layer is larger than a length of the first field platetoward an outer circumferential side of the first clip layer.
 10. Thesemiconductor device according to claim 8, wherein the second clip layeris covered with the first field plate over an insulating film.
 11. Thesemiconductor device according to claim 1, wherein an outercircumferential side end portion of the VLD region is located outside anouter circumferential side end portion of the first electrode.
 12. Thesemiconductor device according to claim 8, further comprising a secondfield plate which is provided in the stopper layer so that the firstfield plate is separated from the second field plate.
 13. Thesemiconductor device according to claim 1, wherein a pn junctionboundary between the VLD region and the semiconductor substrate has awavy shape toward the outer circumferential side.